A hydrogen storage alloy is an alloy capable of safely and easily storing hydrogen as an energy source. Accordingly, the alloy has drawn lots of attention as a new energy conversion and storage material.
Application fields of the hydrogen storage alloy as a functional material are proposed in a wide range such as storage and transportation of hydrogen, storage and transportation of heat, heat-mechanical energy conversion, separation and refining of hydrogen, separation of hydrogen isotopes, batteries using hydrogen as an active mass, catalysts for synthetic chemistry, and temperature sensors.
For instance, a nickel-hydrogen storage battery using a hydrogen storage alloy as a negative electrode material has following characteristics; (a) having a high capacity; (b) being hardly deteriorated even if supercharged or super discharged; (c) being capable of charging and discharging at high efficiency; and (d) causing no bad effect on the environment and is clean. Therefore, the battery has drawn attention as a consumer battery and its applications and practical uses have been actively promoted.
As described above, since the hydrogen storage alloy has capabilities for various applications in terms of mechanical, physical, and chemical properties, it is listed as one of important materials in future industrial fields.
As an electrode material for a nickel-hydrogen storage battery, which is one application example of such a hydrogen storage alloy, are practically used AB5 type rare earth-Ni type alloys having a CaCu5 type crystal structure. However, the discharge capacity of the alloy is limited to about 300 mAh/g and it is difficult to further increase the capacity in the present state.
On the other hand, recently, rare earth-Mg—Ni type alloys, with which capacity increase is made possible, have drawn attention. These alloys respectively have different and complicated layered structures and it is reported that these alloys show discharge capacities exceeding those of the AB5 type alloys in the case of using them for an electrode. For instance, (1) Japanese Patent No. 3015885 (Patent Document 1) discloses electrodes containing LaCaMgNi9 alloys having a PuNi3 type crystal structure. (2) Japanese Patent Application Laid-Open No. 11-323469 (Patent Document 2) discloses that electrodes containing rare earth-Mg—Ni type alloys having a CeNi3 type, Gd2Co7 type, or Ce2Ni7 type structure show a good hydrogen releasing property while keeping high hydrogen storage capacities. (3) Japanese Patent No. 3490871 (Patent Document 3) discloses that particles of hydrogen storage alloys having a Ce5Co19 type crystal structure phase in the surface layer parts and whose composition is defined by a general formula ABx (x is 3.5 to 5) have a high reaction speed with hydrogen in a hydrogen absorption and desorption process.
Patent Document 1: Japanese Patent No. 3015885
Patent Document 2: Japanese Patent Application Laid-Open No. 11-323469
Patent Document 3: Japanese Patent No. 3490871